EP1275622A1

EP1275622A1 - An optical glass for molding

Info

Publication number: EP1275622A1
Application number: EP02254205A
Authority: EP
Inventors: Masaaki c/o Sumita Optical Glass Inc. Otsuka; Yoshinori C/o Sumita Optical Glass Inc Yamamoto; Koichi c/o Sumita Optical Glass Inc. Tsuchiya; Naruhito Sawanobori; Shinobu C/O Sumita Optical Glass Inc. Nagahama
Original assignee: Sumita Optical Glass Inc
Current assignee: Sumita Optical Glass Inc
Priority date: 2001-07-13
Filing date: 2002-06-17
Publication date: 2003-01-15
Anticipated expiration: 2022-06-17
Also published as: JP4818538B2; DE60200080D1; JP2003026439A; HK1048800A1; US6784128B2; US20030153450A1; DE60200080T2; EP1275622B1

Abstract

The present invention provides an optical glass for press molding, in particular, a low softening point glass which contains, in an oxide glass of phosphate type, a durability improving component in addition to glass forming components, and has a weight loss of at most 0.15 weight % in a durability test, and which is represented, in term of elements for making up the glass, by the following chemical composition (mol %): P₂O₅ 32 to 40 % Li₂O 6 to 21 % Na₂O 8 to 31 % K₂O 4 to 22 % Al₂O₃ 7.4 to 16 % ZnO 0 to 19.6 % BaO 0 to 2 % and Sum of Li₂O + Na₂O + K₂O 35.1 to 49 %

Description

This invention relates to an optical glass for molding, in particular, an optical glass for a precision molding lens capable of carrying out molding at no more than 400°C.
Of late, aspherical lenses or micro optical lenses used in optical lens systems have often been produced by a molding technique using a high precision mold without polishing. However, the quality of a metallic mold suitable for molding is subject to various limitations from the aspect of workability, durability and mass producibility. This indicates that the property of a glass to be molded is also limited. The most important property limited is the softening temperature. Molding of a glass having a softening temperature of 600 to 700°C or higher, for example, has a large influence upon the life of a metallic mold and thus results in lowering of the mass producibility of lenses. Accordingly, it has been considered difficult from the standpoint of mass productivity to press-mold commercially available optical glasses of all kinds which have been marketed and consequently, it becomes a subject of research to develop a glass excellent in press moldability.
In JP-A-02-12473, for example, there is disclosed a low softening point, medium refractive index and low dispersion optical glass for a precision molded lens, having a yielding temperature (At) of at most 500°C, refractive index (nd) of 1.53 to 1.62 and Abbe number (vd) 59.0 to 64.0, and comprising P₂O₅ and Z_nO, as an essential element, and 28 to 49 weight % of ZnO + BaO + SrO + CaO + MgO. This optical glass has the feature that grinding or polishing after molding is not required because of a low yielding temperature (At) and excellent stability, chemical durability, weather proofness as well as softening characteristics.
JP-A-08-183632 and JP-A-11-139845 have made similar proposals, with which lowering of the softening temperature of the glass is a common subject.
Many of these glasses, however, have softening temperatures of about 400-500°C, since if lower than this temperature range, there arises a problem that chemical durability is reduced and no practical glass is obtained. When the composition of such a glass is selected so as to correspond to the optical properties of commercially available optical glasses, a sufficient softening temperature sometimes cannot be obtained. Further, a glass described in JP-A-57-027941 is known as a low softening point glass (having a low glass transition temperature). This fluorophosphate glass is a glass having a glass transition temperature of about 100°C, i.e. a very low softening point glass, but it meets with low productivity because it contains a large amount of fluorides resulting in vaporization of low boiling point fluorides during glass melting. Thus, this glass is not considered suitable for mass production.
Phosphate glasses having relatively lower softening temperatures of oxide glasses have hitherto been proposed as a low softening point glass, for example, as shown in JP-A-60-171244, JP-A-61-036136, JP-A-02-116642, JP-A-02-124743, JP-A-03-040934, JP-A-05-132339, JP-A-08-183632, JP-A-09-278479 and JP-A-09-301735.
The inventors have made efforts to develop a glass capable of being subjected to press molding at a low temperature, i.e. at most 400°C, in particular, about 380°C in the above described oxide glass compositions of the phosphate type. Based on the technical common general knowledge of the prior art, it can be considered to increase the alkali component so as to lower the glass softening temperature, but if the alkali component is merely increased, the stability, chemical durability, weather resistance and softening characteristics of the glass cannot be satisfied.
It was found as a result of the Applicant's studies that in the above described glass compositions of the phosphate type, the above described problem can be solved by incorporating a substantial amount of Al₂O₃ in an increased amount of Li₂O, Na₂O and K₂O as an essential element and the present invention is based on this finding.
It is an object of the present invention to provide an optical glass for molding, which is excellent in workability and capable of performing precision molding at a low temperature, e.g. at most 400°C.
It is another object of the present invention to provide, with excellent producibility, an optical glass having optical properties, i.e. refractive index (nd) of 1.50 to 1.55 and Abbe number (vd) of 58 to 67.

Specifically, according to the present invention, there is provided an optical glass for precision moldings, which is represented, in term of elements for making up the glass, by the following chemical composition (mol %):

			Preferred Ranqe
(1)	P₂O₅	32 to 40%	32 to 37%
	Li₂O	6 to 21%	6 to 19.5%
	Na₂O	8 to 31%	12 to 22%
	K₂O	4 to 22%	7 to 19%
	Al₂O₃	7.4 to 16%	8 to 15%
	ZnO	0 to 19.6%	1 to 11%
	BaO	0 to12%	0.5 to 9%
				and
	Sum of Li₂O + Na₂O + K₂O
		35.1 to 49%	36 to 47.5%
(2)	P₂O₅	32 to 37%	32 to 36%
	Li₂O	6 to 19.5%	7 to 14%
	Na₂O	12 to 22%	12 to 19%
	K₂O	7 to 19%	7 to 14%
	Al₂O₃	8 to 15%	9 to 14%
	ZnO	1 to 11%	4 to 11%
	BaO	0.5 to 9%	2 to 9%	and
	Sum of Li₂O + Na₂O + K₂O
		36 to 47.5%	36 to 43.5%

(3) An optical glass for molding, as described in (1) or (2) above, wherein the following components are added (mol %):

MgO	0 to 2%
CaO	0 to 6%
SrO	0 to 2%
Ln₂O₃	0 to 2%	(Ln = La, Gd, Y)
In₂O₃	0 to 2%
ZrO₂	0 to 4%
WO₃	0 to 2%
Nb₂O₅	0 to 2%	and
TiO₂	0 to 1.5%

The low softening point glass according to the present invention is a glass of phosphate type which can mainly be used for optical uses and predominantly comprises P₂O₅, Al₂O₃, ZnO, BaO and R₂O (R: alkali metal), and in particular, at least 7.4% of Al₂O₃ is incorporated as a durability improving component, with success, whereby to impart excellent chemical durability and stability which is represented by a weight loss of at most 0.15 weight %, preferably at most 0.05 weight %, in particular, at most 0.03 weight % in a durability test. This glass has a glass transition temperature (Tg) of 300 to 360°C, molding temperature of 350 to 400°C and optical characteristic values i.e. refractive index (nd) of 1.50 to 1.55 and Abbe number (vd) of 58 to 67.
In a Chemical Durability Test employed herein, a glass sample (1.5 x 1.5 x 1.0 cm) is treated in boiled distilled water for 2 hours and during the same time, a weight loss is measured and represented by percent to the initial weight.
The reasons for limiting the composition range (% should be taken as by mol unless otherwise indicated) of each component of this low softening point, optical glass according to the present invention to the above described (1) are as follows:
P₂O₅ is a glass forming component for the low softening point optical glass according to the present invention, which is present in a proportion of 32 to 40%, since if less than 32%, glass formation is difficult, while if more than 40%, the durability is reduced. The preferred range is 32 to 37%.
Li₂O is a component for improving the softening characteristics of the glass and for reducing the softening temperature of the glass. If the proportion thereof is less than 6%, the above described effect is not sufficient, while if more than 21%, durability and stability are deteriorated. Na₂O is a component for improving the softening property of the glass and for reducing the softening temperature, similar to Li₂O. If the proportion thereof is less than 8%, the above described effect is not sufficient, while if more than 31%, the durability is reduced so as to degrade the stability. K₂O is a component for improving the softening property of the glass and for lowering the softening temperature of the glass, but not so much as Li₂O and Na₂O. If the proportion thereof is less than 4%, the above described effect is not sufficient, while is more than 22%,the durability is reduced so as to render the glass unstable.
The preferred ranges of Li₂O, Na₂O and K₂O are respectively 6 to 19.5%, 12 to 22% and 7 to 19%.
Furthermore, R₂O (Li₂O + Na₂O + K₂O) represents the sum of alkali metal oxide components and is generally adjusted to 35.1 to 49%. If the proportion is less than 35.1%, it is not sufficient to reduce the softening point of the optical glass for molding according to the present invention, while if more than 49%, not only is the durability significantly reduced, but also the glass is unstable. The preferred range is 36 to 47.5%.
Al₂O₃ has the effect of improving durability as the characteristic component of the glass according to the present invention but the solubility is so degraded thereby that the upper limit should be 16%. Further, if the proportion is less than 7.4%, the durability is markedly reduced. Thus, the proportion of Al₂O₃ is generally in a range of 7.4 to 16%. The preferred range is 8 to 15%.
ZnO is a component for assisting reduction of the softening point, but if exceeding 19.6%, the glass is unstable. The preferred range is 1 to 11%.
BaO is a component for improving the melting property and stability of the glass, but if exceeding 12%, the durability is reduced. The preferred range is 0.5 to 9%.
In the optical glass of the foregoing (2) according to the present invention, the reason for limiting P₂O₅, Li₂O, Na₂O, K₂O, R₂O and Al₂O₃ can be illustrated in a substantially similar manner to the foregoing (1), but ZnO and BaO are essential components and if the proportion of ZnO is less than 1%, the stability of the glass gets worse and the effect of reducing the softening point is not sufficiently obtained, while if exceeding 11%, there arises a problem that the stability of the glass is reduced. The preferred range is 4 to 11%.
BaO is a component for improving the stability and durability, since if the proportion is less than 0.5%, the effect on stability is not sufficient and if exceeding 9%, the durability is significantly reduced. The preferred range is 2 to 9%.
The reasons for limiting the optional components in the foregoing (3) are as follows:
RO (oxides of alkaline earth metals selected from Mg, Ca and Sr) is a component for improving the melting property of the glass, but if exceeding the upper limit, the glass is unstable.
In₂O₃ is a component for improving the durability, but if its proportion exceeds 2%, the melting property is degraded so as to retain a residue.
ZrO is a component for significantly improving stability and durability, but if its proportion exceeds 4%, the melting property is degraded so as to retain a residue. The preferred range is 0 to 3.5%.
WO₃ is a component for improving weather resistance, but if the reducing atmosphere is strengthened the glass tends to be colored; accordingly, the upper limit should be 2%.
Nb₂O₅ is a component for improving stability and durability, but its upper limit should be 2%, in order to raise the glass transition temperature.
TiO₂ is a component for improving durability, but since it tends to be colored and crystallized, its upper limited should be 1.5%.
Ln₂O₃ (Ln: La, Y, Gd) is added in a proportion of 0 to 2% so as to improve mainly the weather resistance. If exceeding the upper limit, there arises a problem that the melting property gets worse so as to retain a melting residue.
As illustrated above, the inventors have tried to develop a glass of oxide type, which can be subjected to press molding at a temperature of at most 400°C and have reached an epoch-making glass composition. Such a glass has markedly improved mass productivity and workability in a siding board heating molding (mold pressing) and moreover, gives an improved producibility in a softening production step because of being an oxide type glass. According to the present invention, it is found that press molding a micro optical element having been considered difficult can be carried out with high producibility.
Production of the low softening point optical glass according to the present invention is carried out by a conventional glass production process, using as a raw material, ordinary glass raw materials such as phosphorus pentoxide, salts such as metaphosphates, sodium carbonate and potassium carbonate. A transparent glass can be prepared by adequately melting these raw materials in a platinum crucible at a temperature of about 1000 to 1300°C and then casting the resulting melt into a mold made of carbon, etc. Then, the resulting glass is subjected to annealing at about glass transition temperature, thus obtaining a thermally stable glass. This process can also be applied to production of a preform of a drop of molten glass, which is to be used for the production of a lens for molding.
In these glasses, the glass transition temperature is low, for example, about 300 to 350°C and molding is carried out at about 350 to 400°C. The chemical durability thereof can be represented by a weight loss with a boiling treatment in distilled water in a range of at most 0.15%, which does not constitute any problem in practical use.
A small amount of a defoaming agent such as Sb₂O₃, etc. can further be added.
The following examples are given in order to illustrate the present invention in detail without limiting the same.

(Examples 1 to 43)

Using the corresponding metaphosphates, oxides, carbonates, nitrates, etc., as a raw material of each component, the component compositions (100 g) shown in Tables 2, 5, 8, 11 and 14 were respectively weighed so as to give compositions as shown in Tables 1, 4, 7, 10 and 13 after vitrification, adequately mixed, then charged in a platinum crucible, held molten for several hours in an electric furnace at a temperature of 1000 to 1300°C, homogenized and refined by stirring and then poured into a metallic mold, followed by gradually cooling, to obtain a clear and homogeneous glass.
In Tables 3, 6, 9, 12 and 15 are shown the thermal properties (transition temperature (Tg), yield temperature (At), thermal expansion coefficient at 50-250°C), optical properties (refractive index (nd), Abbe number (vd)) and data of the Chemical Durability Test of the resulting glasses.
In the Chemical Durability Test employed herein, a glass sample (1.5 x 1.5 x 1.0 cm) is treated in boiled distilled water for 2 hours, during which the weight loss is measured and represented by percent to the initial weight.
Thermal properties (transition temperature (Tg), yield temperature (At), thermal expansion coefficient at 50-250°C) were measured by the use of a thermomechanical analyzer (TMA) while raising the temperature at 5°C per minute.

(Comparative Example 1)

As Comparative Example, compositions were optionally chosen within the scope as claimed in JP-B-07-025567.
Using the corresponding metaphosphates, oxides, carbonates, nitrates, etc., as a raw material of each component, these materials were weighed to give 100g of the component compositions as shown in Table 14, Comparative Example 1, after vitrification, adequately mixed, charged in a platinum crucible, held molten for several hours in an electric furnace at a temperature of 1000 to 1300°C, homogenized and refined by stirring and then poured into a metallic mold to obtain a clear and homogeneous glass.
In the similar manner to the Examples, in Table 15 are shown the thermal properties (transition temperature (Tg), yield temperature (At),thermal expansion coefficient at 50-250°C), optical properties (refractive index (nd), Abbe number (vd)) and data of the Chemical Durability Test of the resulting glasses.

Advantages of the Present Invention

The low softening point optical glass with each of the compositions as illustrated above according to the present invention is particularly useful for molding of lenses for molded or micro optical elements with complicated shapes, because of a relatively low softening temperature. Furthermore, this optical glass is more useful due to its chemical durability.

Claims

An optical glass for molding, which is represented, in term of elements for making up the glass, by the following chemical composition (mol %): P₂O₅ 32 to 40 % Li₂O 6 to 21 % Na₂O 8 to 31 % K₂O 4 to 22 % Al₂O₃ 7.4 to 16 % ZnO 0 to 19.6 % BaO 0 to 12 % and Sum of Li₂O + Na₂O + K₂O 35.1 to 49 %
An optical glass for molding, which is represented, in term of elements for making up the glass, by the following chemical composition (mol %): P₂O₅ 32 to 37 % Li₂O 6 to 19.5 % Na₂O 12 to 22 % K₂O 7 to 19 % Al₂O₃ 8 to 15 % ZnO 1 to 11 % BaO 0.5 to 9 % and Sum of Li₂O + Na₂O + K₂O 36 to 47.5 %
An optical glass for molding, as claimed in claim 1 or claim 2, wherein the following components are added thereto (mol %): MgO 0 to 2 % CaO 0 to 6 % SrO 0 to 2 % In₂O₃ 0 to 2 % ZrO₂ 0 to 4 % WO₃ 0 to 2 % Nb₂O₅ 0 to 2 % TiO₂ 0 to 1.5 % La₂O₃ 0 to 2 % Y₂O₃ 0 to 2 % and Gd₂O₃ 0 to 2 %